From: Jann Horn <jannh@xxxxxxxxxx> [ Upstream commit 2c2ade81741c66082f8211f0b96cf509cc4c0218 ] The basic idea behind ->pagecnt_bias is: If we pre-allocate the maximum number of references that we might need to create in the fastpath later, the bump-allocation fastpath only has to modify the non-atomic bias value that tracks the number of extra references we hold instead of the atomic refcount. The maximum number of allocations we can serve (under the assumption that no allocation is made with size 0) is nc->size, so that's the bias used. However, even when all memory in the allocation has been given away, a reference to the page is still held; and in the `offset < 0` slowpath, the page may be reused if everyone else has dropped their references. This means that the necessary number of references is actually `nc->size+1`. Luckily, from a quick grep, it looks like the only path that can call page_frag_alloc(fragsz=1) is TAP with the IFF_NAPI_FRAGS flag, which requires CAP_NET_ADMIN in the init namespace and is only intended to be used for kernel testing and fuzzing. To test for this issue, put a `WARN_ON(page_ref_count(page) == 0)` in the `offset < 0` path, below the virt_to_page() call, and then repeatedly call writev() on a TAP device with IFF_TAP|IFF_NO_PI|IFF_NAPI_FRAGS|IFF_NAPI, with a vector consisting of 15 elements containing 1 byte each. Signed-off-by: Jann Horn <jannh@xxxxxxxxxx> Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> --- mm/page_alloc.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 3af727d95c17..05f141e39ac1 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -3955,11 +3955,11 @@ void *__alloc_page_frag(struct page_frag_cache *nc, /* Even if we own the page, we do not use atomic_set(). * This would break get_page_unless_zero() users. */ - page_ref_add(page, size - 1); + page_ref_add(page, size); /* reset page count bias and offset to start of new frag */ nc->pfmemalloc = page_is_pfmemalloc(page); - nc->pagecnt_bias = size; + nc->pagecnt_bias = size + 1; nc->offset = size; } @@ -3975,10 +3975,10 @@ void *__alloc_page_frag(struct page_frag_cache *nc, size = nc->size; #endif /* OK, page count is 0, we can safely set it */ - set_page_count(page, size); + set_page_count(page, size + 1); /* reset page count bias and offset to start of new frag */ - nc->pagecnt_bias = size; + nc->pagecnt_bias = size + 1; offset = size - fragsz; } -- 2.19.1
